Electrochemical production of flucrocarbon acid fluoride derivatives



2,71 7 ,8 7 1 Patented Sept. 13, 1955 ELECTROCHEMICAL PRODUCTION OFFLUGRO- CARBON ACID FLUORIDE DERIVATIVES Harold M. Scholberg, St. Paul,Minn., and Hugh G. Bryce, Hudson, Wis., assignors to Minnesota Mining &Manufacturing Company, St. Paul, Minn., a corpo ration of Delaware NDrawing. Application February 1, 1952, Serial No. 269,584

3 Claims. (Cl. 204-59) This invention relates to our discovery of a newand useful process of making saturated fluorocarbon acid fluorides,which are converted to derivatives thereof and recovered as such. It isan improvement upon the electrochemical procedures described in the U.S. patents of J. H. Simons, No. 2,519,983 (August 22, 1950), and A. R.Diesslin, E. A. Kauck and J. H. Simons, No. 2,567,011 (September 4,1951), and further described in a paper by E. A. Kauck and A. R.Diesslin, published by the American Chemical Society in Industrial andEngineering Chemistry, vol. 43, pp. 2332-2334 (October 1951).

These references describe an electrochemical fluorination process ofmaking saturated fluorocarbon acid fluorides, which (in the case ofmonoc'arboxylic acid fluorides) can be represented by the formulas RtCOFor /0 RiC where Rf stands for a saturated fluorocarbon group (cyclic ornon-cyclic) consisting solely of carbon and fluorine. The non-cyclic(aliphatic) compounds have the formula: CnF2n+1COR and the cycliccompounds have the formula: CnFZn-ICOF. These compounds may also betermed saturated perfiuorocarboxylic acid fluorides, and saturatedperfluoroacyl fluorides.

The procedure described in these references involves electrolyzing acurrent-conducting solution comprising anhydrous liquid hydrogenfluoride to which has been added a hydrocarbon carboxylic acid (or itsanhydride), by passing direct-current through the solution at a cellvoltage which is insuflicient to generate molecular (free elemental)fluorine under the existing conditions, but which is suflicient to causethe formation of the desired fully fluorinated acid fluoride at a usefulrate. Use is made of a single-compartment cell without diaphragms. Theelectrode pack consists of alternating and closely-spaced iron andnickel plates, serving as cathode and anode electrodes, respectively.The cell can be conveniently operated at substantially atmosphericpressure and at temperatures in the neighborhood of 0 to C. The appliedcell voltage is approximately 5 to 6 volts.

The fluorocarbon acid fluoride product of the cell operation isrelatively insoluble in the electrolyte solution and either settles tothe bottom of the cell from which it can be drained with otherfluorocarbon products of the process, or is volatilized and evolves fromthe cell in admixture with the hydrogen and other gaseous products,depending upon its volatility. The fluorocarbon acid fluoride compoundsare very reactive and the normal procedure is to promptly convert themto derivatives without isolating them first in pure form, and recoveringthe derivatives. A cell product mixture containing the fluorocarbon acidfluoride can be hydrolyzed with water to produce the correspondingfluorocarbon acid (RrCOOH), or can be reacted with ammonia to producethe amide (RrCONHz), or can be reacted with an alcohol to produce anester (RfCOOR) for example. The derivative can then be recovered in ell)fluorides, the hydrogen atoms and the pure form by a suitable procedure.Numerous other derivatives can be made from these initial derivatives.

Unsaturated acids as wellas saturated acids can be used as startingcompounds and saturation is produced by fluorine addition during theelectrochemical fluorination.

The electrochemical process is not limited to the production ofmonocarboxylate compounds. The hydrocarbon polycarboxylic acids (andtheir anhydrides) can be fluorinated to produce the correspondingfluorocarbon acid hydroxyl groups of the starting acid being replaced byfluorine atoms. The fluorocarbon acid fluorides can be genericallyrepresented by the formula:

Where m is an integer having a value of 1 for monocarboxylic acidfluorides, a value of 2 for dicarboxylic acid fluorides, etc.

The process as heretofore described and used, outlined above, has theeconomic disadvantage of producing relatively low yields of thefluorocarbon acid fluoride compound corresponding to the hydrocarbonacid (or its anhydride) used as the starting compound. Even in the mostfavorable case, the production of trifluoroacetyl fluoride (CFsCOF) fromacetic acid (CH3COOH) or its anhydride, the consumed acid startingcompound is less than 50% converted to CFaCOF, due to molecularfragmentation and partial fluorination resulting in substantial yieldsof CR1, CFsH, 0P2, COFz and CO2. In the case of higher acids, stillother by-product compounds are produced and the yield of the desiredacid fluoride (corresponding to the starting compound) decreases rapidlywith increase in number of carbon atoms. The situation is even moreunfavorable in the case of polycarboxylate compounds. The yields whenusing hydrocarbon acids as starting compounds are materially lower thanwhen using the anhydrides of the acids as starting compounds. Theseobservations are based on a great many laboratory and pilot plant runsby Minnesota Mining & Manufacturing Company (St. Paul, Minnesota)wherein numerous operating variables and expedients have been studied inthe attempt to improve yields.

The importance of this from the commercial production standpoint isapparent in view of the high prices which it has been necessary tocharge for fluorocarbon compounds and which have seriously limited theiracceptance except for special applications. charge from $10.00 per poundto $50.00 per pound and upwards for fluorocarbon acids.

It is evident, therefore, that any innovation Which can materiallyincrease the yields of the electrochemical process is of great value inpromoting the usage of fluorocarbon compounds, which are unique and havemany fields of utility that could be served if not too expensive.

We have discovered a modification of the above-described process by whicas the result of using different starting compounds, the yields oftrifluoroacetyl fluoride can be substantially doubled, and the yields ofhigher compounds can be improved in an even greater ratio, as comparedwith the yields obtained when using the anhydrides of hydrocarbon acidsas starting compounds. The improvement is even more marked whencomparison is made with the use of hydrocarbon acids as startingcompounds. Furthermore, the acid fluoride product yield per unit ofelectrical energy (electrical efficiency) is more than doubled. Afurther advantage of the present procedure is that there is no formationof OF2 (oxygen fluoride), apart from what may be formed from impurities,and the formation of COFz (caronbyl fluoride) is markedly decreased.

In this new procedure, we employ as the starting compound the acidfluoride of the hydrocarbon carboxylic acid, that is, the hydrocarbonacyl fluoride, rather than It has been necessary to 3 the acid itself(or its anhydride). Despite the higher cost of the acid fluoride ascompared with the acid (or its anhydride) there is a very substantialnet economic gain because of the extent of gain in the yield and in theelectrical efficiency.

According to one procedure the hydrocarbon acid fluoride startingcompound is directly added to the liquid HF of the cell. It is convertedby the electrochemical process to the corresponding fluorocarbon acidfluoride compound as indicated by:

where R is a hydrocarbon group (saturated or unsaturated), R: is thecorresponding saturated fluorocarbon group (resulting from completefluorination), and m is an integer.

Another procedure is to add the corresponding hydrocarbon acid chloridecompound to the liquid HF (either before or after the latter isintroduced into the cell), whereupon reaction occurs (even when nocurrent is flowing) by which the chlorine atoms are replaced by fluorineatoms, with evolution of HCl, resulting in a solution of hydrocarbonacid fluoride in the liquid HP. The HCl is insoluble in the liquid HFand is released as fast as formed. As before, electrochemicalfluorination then results in the production of the fluorocarbon acidfluoride product. These two steps can be indicated by:

This two step procedure has an advantage in many cases since the acidchloride compound may be more readily or cheaply prepared from theoriginal source materials. In either case, the hydrocarbon acid fluoridecompound is used as the actual starting compound (dissolved in theliquid HF) that is electrochemically fluorinated. Similarly, use can bemade of the hydrocarbon acid bromide or iodide compound, which likewisereact in liquid HF to yield the acid fluoride compound, releasing HBr orHI (which are low-boiling gases insoluble in the HF).

Thus the starting compound added to the liquid HF is in any case ahydrocarbon acid halide, R(COX)m, where X stands for F, Cl, Br or I.When the chloride, bromide or iodide is added it is converted to thefluoride, which is the actual starting compound for the electrochemicalprocess.

Pure anhydrous liquid HF is non-conductive. Hydrocarbon acid fluorides(acyl fluorides) such as acetyl fluoride (CHzCOF) and butyryl fluoride(CaHvCOF) are soluble but do not ionize therein, and therefore a pureanhydrous solution is non-conductive. This is in contrast to thecorresponding acids and their anhydrides which even in pure form can beadded to pure anhydrous liquid HF to provide conductive solutions. Thenecessary conductivity of the solution to permit of eflicient currentflow in the electrochemical cell, can be provided by including a smallamount (e. g., 0.1 to 5%) of sodium fluoride as a conductivity additive(carrier electrolyte). The use of conductivity additives in conjunctionwith non-conductive organic starting compounds was described in theaforesaid patentof I. H. Simons, No. 2,519,983 (see, especially, columns9-11), and need not be elaborated upon. A small amount of acid or atrace of water can also be employed for this purpose, for example. Infact, the materials employed in practicing the present process willoften be found to contain impurities (such as traces of acid or water orboth) which will serve as carrier electrolytes and make unnecessary thedeliberate addition of a conductivity additive.

As shown by Example 4, adequate conductivity has been obtained withoutusing a conductivity additive in the case of starting compoundscontaining a substantial number of carbon atoms in the molecule. Thescientific explanation is in doubt, since it may be that the highercompounds ionize sufliciently to provide adequate conductivity ductivityadditive, since previously it had been the experience that thenon-ionizable organic starting compounds (that require the use of anadditive to provide a carrier electrolyte) cannot be electrochemicallyfluorinated in as high yields and efliciencies as can those which ionizein the IF and per se provide adequate conductivity. 3. H. Simons hadpublished a negative report on evidence of formation of trifluor'oacetylfluoride (CFsCOF) by the electrochemical process, using acetyl chloride(CH3COCI) as the starting compound in conjunction with sodium fluorideto provide conductivity (J. H. Simons et al.,

Journal of the Electrochemical Society, vol. 95, No. 2, February 1949,pp. 4767, see especially pp. 5354). The particular circumstances of hislaboratory experiment were quite different from the operating conditionsand procedures of the electrochemical process as employed in our workand in plant operations.

The reality of the substantial increase in yield obtained by ourprocess, and of the reduction in manufacturing cost, has beendemonstrated by many laboratory experiments and pilot plant runs whereinall other variables were kept as constant as possible in makingcomparisons. The improvement is of too high a magnitude to be explainedby any variations in other conditions. Moreover, the improvement wasfound to be present even when large variations were made in operatingconditions; such as variations in concentration of organic startingmaterial ranging from /2% to 30%, variations in concentration of sodiumfluoride (or other conductivity additive or carrier electrolyte),variations in temperature ranging from 10 to 40 C., etc. (It is notmeant to imply that these are operative limit ranges; they merelyindicate a range of experimentation which has established that theimprovement is not limited to some particular combination of operatingconditions.)

Experiments have been run using alkyl monocarboxylic acid fluoridestarting compounds (C11H27L+1COF) having from two to ten carbon atoms inthe molecule to produce the corresponding fluorocarbon acid fluorides(CnF21H-1COF) having from two to ten carbon atoms in the molecule; whichwere hydrolyzed to produce the corresponding acids (CnF2n+1COOH),ranging from trifluoroacetic acid (CFaCOOH, having a B. P. of 72 C.) toperfluorocapric acid (C9F19COOH, having a B. P. of 218 C.). Both thenormal and the iso-perfluorobutyric acids were made in this way, usingbutyryl fluoride and isobutyryl fluoride, respectively, as startingcompounds. The unsaturated starting compound crotonyl fluoride, CxHsCOF,was also used in making perfluorobutyryl acid fluoride, CsFwCOF, whichwas hydrolyzed to yield perfluorobutyric acid, CIZF'ICOOH.

In an experiment using phthalyl fluoride, CsH4(COF)2, as the startingcompound, a mixture of two different acid fluoride product compounds wasobtained, namely, perfluorocyclohexane-dicarboxylic acid fluoride,CsF1o(COF)2, and perfluorocyclohexanecarboxylic acid fluoride, CeFuCOF;and these were hydrolyzed to produce the corresponding acids,perfluorocyclohexane-dicarboxylic acid, CeF1o(COOH)z, andperfluorocyclohexane-carboxylic acid, CsFuCOOI-I.

Examples of aliphatic fluorocarbon polycarboxylic acids that have beenmade by employing the present invention are perfluorosuccinic acid,(CF2)2(COOH)2, produced from CFz) 2 (GOP) 2 using succinyl fluoride,(CH2)2(COF)2, as the starting compound; perfluoroadipic acid(CF2)4(CO0H)2, produced from (CF2)4(COF)2 using adipyl fluoride,(CH2)4(COF)2, as the starting compound; and perfiuorosebacic acid,(CF2)8(COOH)2, produced from (CF2)'8(COF)2 using sebacyl fluoride,(CH2)s(COF )2, as the starting compound. Perfluorosebacic acid wasdescribed and claimed in a copending application of R. A. Guenthner,since issued as Patent No. 2,606,206 (August 5, 1952 Our process hasparticularly notable commercial value in making acids containing fromfour to ten carbon atoms in the molecule, which constitute an importantclass as to utility and as to which the previously obtained low yieldsstood as a strong obstacle to extensive commercial use.

Example 1' Use Was made of an iron-cathode nickel-anode pilot plant cellwhich had an anode surface area of about 110 square feet. (A photographof this pilot plant cell appears on page 418 of the book FluorineChemistry, edited by J. H. Simons, published in 1950 by Academic PressInc., New York City.) The cell was initially charged with 13 pounds ofacetyl fluoride (CHzCOF) and about 330 pounds of anhydrous liquid HP, towhich 7.5 pounds of sodium fluoride were added as a carrier electrolyte.The CHsCOF and HF were replenished from time to time during the run tosubstantially maintain the initial concentration. The cell was operatedat a pressure of about 3 p. s. i. gauge (i. e., slightly aboveatmospheric pressure) and the cell temperature was about C. The averageconcentration of CI-IaCOF was about 4.5%. The average current value was1955 amperes and the average voltage value was 5.45 volts. The averagecurrent density was 18 amperes/sq. ft. The duration of the run was 1145hours.

The gas mixture from the cell was led through a series of lowtemperature condensers to condense out the bulk of the HF which wasdrained back to the cell. The exit gas mixture, after warming to roomtemperature, was passed through a packed tower countercurrently to adescending flow of water. The bottom eflluent was an aqueous solutioncontaining the CFsCOOH and HF formed by hydrolysis of the CFaCOF. Thiswas processed to recover the trifluoroacetic acid (CFzCOOH) in pureform.

During the run 1750.5 pounds of acetyl fluoride were consumed (thisbeing the total amount added to the cell for replenishment). A total of2276.9 pounds of trifluoroacetic acid was produced, determined fromanalysis of the eflluent solution from the tower. The conversion of theCFsCOF to CFaCOOH was essentially quantitative. The average productionrate of CFaCOOI-I was 1.015 pounds per 1000 ampere hours. The yield ofacid (and hence also the yield of CFaCOF) was 71% based upon the acetylfluoride charged to the cell.

A closely similar run was made using acetic anhydride, (CI-I3CO)2O, asthe starting compound charged to the cell, and provides a comparison. Inthis case the production rate of CFsCOOH was 0.484 pound per 1000 amperehours. The yield of acid (and hence also the yield of CF3COF) was 38%based upon the acetic anhydride charged to the cell.

Example 2 Example 3 In this experiment the Example 1. pounds ofprocedure was similar to that of The pilot plant cell was charged with13 n-butyryl fluoride, CH3(CH2)2COF, 330

pounds of anhydrous liquid HF, and 0.9 pound of NaF. The averageconcentration of the n-butyryl fluoride was 5% and a total of 444 poundswas added during the run of 711 hours. The current averaged 1670amperes, the voltage averaged 5.9 volts, and the average current densitywas about 15 amperes per sq. ft.

A total of 416 pounds of crude acid was produced which analyzed 93%CF3(CF2)2COOH, and 7% CH3 (CFz zCOOH as the starting compound. A totalof 907 pounds was added for replenishment during the run. A total of522.8 pounds of crude acid was produced which analyzed 65%CF3(CF2)2COOH, and 35% CFsCFzCOOH and CFsCOOH. The average productionrate of the heptafluorobutyric acid was 0.144 pound per 1000 amperehours. The yield was 15.3% based on the n-butyric acid charged. to thecell.

This comparison shows how strikingly the present process increases theacid fluoride production rate per 1000 ampere hours (electricalefficiency), the relative yield of the desired acid versus lowerby-product acids, and the yield of acid relative to starting compoundemployed.

Example 4 The same pilot plant cell was used in this run. The initialcharge was 350 pounds of anhydrous liquid HF and 35 pounds of caprylylchloride, CH3(CH2)sCOCl, both of which were replenished during the runof 537 hours. A total of 442 pounds of additional caprylyl chloride wasadded. The additions of caprylyl chloride were made slowly and the cellwas vented to release the HCl which instantly forms and is insoluble inthe HF, so as to avoid an explosive reaction. The cell was operated at apressure of 6 p. s. i. gauge and at a temperature of about 24 C. Anaverage current of 1650 amperes at 5.8 volts was passed through thecell. No conductivity additive was found necessary due, possibly, to thepresence of impurities which performed the function of a carrierelectrolyte, or, possibly, to ionization of this starting compound, orboth.

The liquid product, consisting of a mixture of fluorocarbons and offully fluorinated acid fluorides, was drained from the cell and fromtraps located beneath the low-temperature condensers, and totalled 745.1pounds. This material was treated with water to hydrolyze the acidfluorides, and the perfluorocaprylic acid CF; (CF2)eCOOH was obtainedwhen using adipyl chloride, (CH2)4(COC1)2, rather than adipic acid,(CH2)4(COOH)2, to charge the cell.

We claim: 1. An electrochemical process of making fluorocarbon acidfluoride derivatives by electrolyzing, in a cell containing an electrodepack having nickel anodes, a currentconducting solution comprisinganhydrous liquid hydrogen fluoride mixed with an appropriate organicstarting compound, the cell being operated at an average temperaturewhich is not greatly below 0 C. and at an average voltage which does notexceed approximately 6 volts, such that a fluorocarbon acid fluorideproduct is obtained in a useful yield, and converting the fluorocarbonacid fluoride product to a derivative thereof which is recovered,characterized by electrolyzing a hydrocarbon acid fluoride startingcompound.

2. An electrochemical process of making fluorocarbon acid fluoridederivatives by electrolyzing, in a cell containing an electrode packhaving nickel anodes, a currentconducting solution comprising anhydrousliquid hydrogen fluoride mixed with an appropriate organic startingcompound, the cell being operated at an average temperature which is notgreatly below 0 C. and at an average voltage which does not exceedapproximately 6 volts, 0

such that a fluorocarbon acid fluoride product is obtained in a usefulyield, and converting the fluorocarbon acid fluoride product to aderivative thereof which is recovered, characterized by adding to theliquid hydrogen fluoride a hydrocarbon acid halide to provide adissolved hydrocarbon acid fluoride and electrolyzing it assubstantially the only organic starting material employed.

5 gen fluoride mixed with an appropriate organic starting compound, thecell being operated at an average temperature which is not greatly below0 C. and at an average voltage which does not exceed approximately 6volts, such that a fluorocarbon acid fluoride product is obtained 10 ina useful yield, and converting the fluorocarbon acid fluoride product toa derivative thereof which is recovered, characterized by electrolyzinga hydrocarbon acid fluoride starting compound having from four to tencarbon atoms in the molecule.

Cited in the file of this patent UiIiTED STATES PATENTS 2,519,983 SimonsAug. 22, 1950 2,567,011 Diesslin et al Sept. 4, 1951 2,606,206 GuenthnerAug. 5, 1952 OTHER REFERENCES Simons et al.: Journal ElectrochemicalSociety, vol. 95

(February 1949), pp. 53-54.

Kauck et al.: Industrial and Engineering Chemistry,

vol. 43 (October 1951), pp. 2332-2334.

1. AN ELECTROCHEMICAL PROCESS OF MAKING FLUOROCARBON ACID FLUIDIZEDERIVATIVES BY ELECTROLYZING, IN A CELL CONTAINING AN ELECTRODE PACKHAVING A NICKEL ANODES, A CURRENTCONDUCTING SOLUTION COMPRISINGANHYDROUS LIQUID HYDROGEN FLUORIDE MIXED WITH AN APPROPRIATE ORGANICSTARTING COMPOUND, THE CELL BEING OPERATED AT AN AVERAGE TEMPERATUREWHICH IS NOT GREATLY BELOW 0* C. AND AT AN AVERAGE VOLTAGE WHICH DOESNOT EXCEED APPROXIMATELY 6 VOLTS, SUCH THAT A FLUOROCARBON ACID FLUORIDEPRODUCT IS OBTAINED IN A USEFUL YIELD, AND CONVERTING THE FLUOROCARBONACID FLUORIDE PRODUCT TO A DERIVATIVE THEREOF WHICH IS RECOVERED,CHARACTERIZED BY ELECTROLYZING A HYDROCARBON ACID FLUORIDE STARTINGCOMPOUND.